The utilization of threaded closures in packaging of carbonated beverages has become very popular. The popularity is due in part to the fact that the consumer can open the package by merely unscrewing the closure from the container. No "bottle opening" tool is needed. Another advantage is that the consumer is able to remove the closure, dispense part of the contents from the container and reclose the container by merely screwing the closure back thereon. Since the sealing system is generally of high fidelity, there will be little loss of carbonation and the remaining packaged product will be suitable for use at a later time.
Despite these advantages, the threaded container-closure package has potentially a serious problem, i.e., premature release of the closure from the container which can occur with great force. The premature release generally occurs as the user unscrews the closure to remove it from the container. Unscrewing of the closure results in lessened thread engagement between the closure and container threads until all engagement is lost and the closure can be removed from the container. Also, the initial unscrewing of the closure results in breaking the seal between the top of the closure and the top of the container. Upon loss of the seal, pressurized gas enters between the sidewall of the closure and the container tending to bulge the closure sidewall outwardly. As the closure sidewall bulges outwardly, the closure threads are pulled away from normal full contact with the container threads. The blow-off or premature release occurs when the pressure of the gas in the container is able to overcome the engagement of the closure thread with the container thread. The bulging out of the closure sidewall compounds the problem as it diminishes thread contact and thus the holding power of the thread engagement.
Venting of the pressurized gas helps reduce the blow-off problem somewhat. Venting can be accomplished, for example, by using a vertical vent slot on the container or closure. With the vent slot, the gas is not trapped between the closure sidewall and the container neck. However, there is still a chance for blow-off if the thread engagement is lessened too quickly as sufficient time will not have passed for the pressurized gas to complete its venting. For some closures, complete unscrewing of the closure from the container can take as little as one-half of a second. Clearly, in this amount of time venting has only started and pressure in the container is still high.
Therefore, it is an object of this invention to provide a threaded closure which, due to its particular features, requires an unscrewing time sufficiently long to provide adequate venting time.